1. Field of the Invention
This invention relates to transistor amplifiers of the Darlington type, and to means for producing bias current therein so as to improve the operating characteristics of such amplifiers, particularly when used in differential configurations in integrated circuits.
In many types of differential amplifiers such as prior art differential amplifier A illustrated in FIG. 1, it is desirable to load a differential input stage I with an active load L1, that is, a load formed by the output impedance of two transistors Q1 and Q2 operated at constant current. An acitve load provides a higher dynamic impedance than could be obtained with a resistor operating at the same current and with the same supply voltage limitations. This higher impedance results in a large first-stage gain, which is desirable for several commonly recognized reasons.
FIG. 1 illustrates the basic configuration of a PNP active load L1. A common-mode loop or other means such as voltage source V is provided to apply a bias voltage to the bases of PNP transistors Q1 and Q2. This voltage causes voltage to be developed across resistors R1 and R2. Assuming R1 = R2 and that the base-emitter voltages Vbe1 and Vbe2 of Q1 and Q2 are equal, the emitter currents of the two transistors will be forced to be equal. As a result, if .beta., the common base current transfer ratio, is the same in both transistors, the collector currents ic.sub.1 and ic.sub.2 will be equal. This equality of collector currents is essential since the collector currents determine the operating currents of the two NPN halves Qa and Qb of the input stage I, and this current ratio strongly influences the input offset voltage of the differential amplifier D at its inputs a, b.
If the two base-emitter voltages Vbe1 and Vbe2 do not ideally match, this effect can be suppressed by increasing the voltage drop across R1 and R2. Therefore, there are means at hand to reduce errors resulting from Vbe mismatch to arbitrarily small values. If the base currents ib.sub.1 and ib.sub.2 do not match, unequal base currents are subtracted from emitter currents which are forced to be equal. As a result, the collector currents will differ by the amount of base current difference. Increasing the transistor current multiplication factor .beta. (or .alpha.) has the effect of diminishing the absolute magnitude of the base currents, and, hopefully, their resulting difference. However, integrated circuit process technology limits the .beta. below values which give satisfactory base current differences.
Efforts to provide a satisfactorily high .beta. may take the form shown in FIG. 2, which illustrates an active load L2 with transistors Q3 and Q4 added to form two Darlington amplifiers D1 and D2.
Transistor amplifiers of the Darlington type are characterized by a construction in which successive cascaded transistors, connected to form a three terminal device, have an overall current multiplication factor .beta. essentially equal to the product of the current multiplication factors of the individual transistors. The successive transistors Q1-Q3 and Q2-Q4 are connected, as shown, e.g., in Darlington U.S. Pat. No. 2,663,806, with the two collectors in common and the emitter of the first transistor connected to the base of the second transistor.
Transistor amplifiers of the Darlington type are advantageous because of their very high current gain and because, as three terminal devices, they can be substituted for single transistors.
As a result in active load L2 the Darlington amplifiers formed by composite transistors Q1-Q3 and Q2-Q4 have very high effective gains .beta. 13 and .beta. 24 which essentially are the products .beta. 1 .beta. 3 and .beta. 2 .beta. 4. This will reduce the base currents substantially. The difference between .beta. 13 and .beta. 24 may, of course, be larger than the difference in the configuration of FIG. 1, so that the improvement may be less than anticipated. This results because integrated circuit techniques are capable of matching current gain in separate transistors on the same chip only to within about 20%. Since the base currents of Q1 and Q2 are assumed to be unequal, the operating currents of Q3 and Q4 will differ. As a result any difference in their .beta.'s will be exaggerated. Moreover, the different operating currents will cause a Vbe mismatch which aggravates the problem of forcing equal emitter currents, particularly since Vbe varies thermally. An additional disadvantage is that the low bias level in Q3 and Q4 results in poor frequency response and dynamic performance.
2. Description of the Prior Art
Various biasing arrangements have been proposed to alleviate the foregoing problems of the Darlington amplifier. One such arrangement, illustrated in FIG. 2, includes two current sources i.sub.3 and i.sub.4 (shown in dotted lines). These currents bias Q3 and Q4 at equal levels, to improve .beta. match and Vbe match, and at a current level which provides satisfactory dynamic performance. The disadvantage of this circuit is the difficulty of designing the current sources for i.sub.3 and i.sub.4. Since most of these currents appear in the active load output, they must be very well matched. Now the problem of matching the original simple active load has been transformed into a problem of matching i.sub.3 to i.sub.4. The matching requirement is less severe, but the complexity involved is high, since i.sub.3 and i.sub.4 themselves may require a circuit as complex as the circuit shown in FIG. 1, if temperature and operating point problems are to be avoided.
In another such arrangement, illustrated in FIG. 3, a Darlington amplifier D3 has a resistor R connected between the emitter of first transistor Q1 and the emitter of second transistor Q2 to bias the first transistor Q1 at a current level determined by the base-emitter voltage Vbe2 of the second transistor Q2 divided by the resistance R. Because integrated circuit techniques provide very good matches between base-emitter voltages Vbe, the first transistor Q1 on each side of the differential pair of Darlington amplifiers will have its collector current level biased uniformly instead of varying as the inverse of the .beta. of the second transistor Q2. This arrangement has not been fully satisfactory, however, because the stability of the bias point of the first transistor Q1 still fluctuates at about 0.5% per degree centigrade along with the thermal fluctuation in Vbe. The fluctuation in bias point is transmitted directly to the offset current which thus is excessively temperature sensitive.